Examples of a single antenna that supports a plurality of frequency bands are disclosed in Patent Document 1 (International Publication No. WO 2006/073034) and International Publication No. WO 2006/077714 (Patent Document 2).
Here, the configuration of the antenna illustrated in Patent Document 1 is described on the basis of FIG. 1. In the example of FIG. 1, a feed radiation electrode 7 is provided on a rectangular columnar dielectric base member 6. This feed radiation electrode 7 resonates in a fundamental mode and a higher mode. The feed radiation electrode 7 has a first end formed as a feed end 7A for use in connection to a circuit for wireless communication. The feed radiation electrode 7 has a second end formed as an open end 7B. The position of a capacitance-loading portion α is set in advance between the feed end 7A and the open end 7B of the feed radiation electrode 7. The capacitance-loading portion α is connected to a capacitance-loading conductor 12. The capacitance-loading conductor 12 produces a capacitance for use in adjusting the resonant frequency in the fundamental mode between the feed end 7A and the capacitance-loading portion α.
In the antenna illustrated in Patent Document 2, a dielectric base member on which a feed radiation electrode and a non-feed radiation electrode are disposed is arranged in an ungrounded area of a substrate, each of the feed and non-feed electrodes having a spiral slit, and capacitance is formed in the spiral slit.
With the antenna illustrated in Patent Document 1, the magnitude of the capacitance connected between the feed end 7A and the capacitance-loading portion α is specified by the capacitance-loading conductor 12. The use of this can adjust the resonant frequency in the fundamental mode. Setting the position of the capacitance-loading portion α in advance enables the adjustment of the resonant frequency in the fundamental mode while the resonant frequency in a harmonic mode remains substantially constant.
However, in order to adjust or change the load capacitance, it is necessary to alter the shape of the electrode pattern on the rectangular columnar dielectric base member. The same applies to the antenna illustrated in Patent Document 2. For example, when it operates as a double-channel antenna for the 2 GHz band and the 900 MHz band, the resonant frequency in the fundamental mode is set as the 900 MHz band and the resonant frequency in the harmonic mode is set as the 2 GHz band. In order to change the resonant frequency in the fundamental mode by using the load capacitance, as well as in order to change the resonant frequency in the harmonic mode, it is necessary to alter the electrode pattern. Because of this, development and design time is required, and a problem also exists in an increase in cost.